Days are not as long as they should be. Scientists know that the moon’s gravity pulls on Earth and slows its rotation, so that yearly, the days get about 20 millionths of a second longer. But Benjamin Chao has discovered a countervailing effect, one that shortens the day by about .2 millionths of a second per year. Chao, a geophysicist at NASA’s Goddard Space Flight Center, says humans have redistributed enough mass on Earth to affect the planet’s rotation rate. How? By storing water in artificial reservoirs, most of which have been built in the past four decades.
Chao estimates that over 2 quadrillion gallons of water that would otherwise be spread evenly in oceans are in reservoirs, lowering sea levels by more than an inch. To estimate the effects of this on Earth’s rotation, Chao assessed the capacity of 88 reservoirs worldwide that hold more than 2 trillion gallons, like Hoover Dam.
Most of the reservoirs are in the Northern Hemisphere, he says, so there has been a net shift of water northward, away from the equator-- and thus closer to Earth’s axis. That makes the planet spin faster, just as figure skaters spin faster when they pull their arms in. The mass shift has also made the North Pole move about two feet toward 130°W longitude-- essentially because the planet is being tipped the other way by a concentration of reservoirs in the Eastern Hemisphere, mostly in Asia. Both effects, Chao says, are probably even greater than he has estimated; the reservoirs he studied hold less than half the water dammed by humans.